1. Field of the Invention
The present invention relates to a bituminous mixture for paving applications. More specifically, this bituminous mixture provides a paved layer with increased vapor permeability while the layer remains substantially moisture impervious and retains its ability to retard the formation of reflective cracks.
2. Description of Related Art
When pavements deteriorate, they may be overlaid with hot mix asphalt (HMA) to repair them. When designing an overlay, the rate of crack propagation through the overlay, the rate of deterioration of the reflective crack, and the amount of water that can infiltrate through the crack must be considered. One disadvantage with such thicker HMA overlays is that cracks in the old pavement reflect through the new overlay. To relieve this reflective cracking, one option is to place thicker overlays. Another disadvantage with conventional HMA overlays is that they are water permeable allowing water to enter the base. A third disadvantage with these overlays is that they typically have a low strain tolerance and a low resistance to reflective cracking.
Asphalt binders that display the ability to undergo creep or stress relaxation at low temperatures in order to minimize the potential for thermal and reflective cracking may be created. The disadvantage with such binders is that they are highly ductile and thus, roads created with them tend to rut.
Asphalt binders with a high shear modulus that resist rutting at high temperatures also may be created. The disadvantage with such binders is that they tend to be brittle at low temperatures, and thus, roads created with them tend to crack. Typical asphalt binders formulated for pavement applications usually display either high shear modulus at high temperatures or high ductilities at low temperatures but not both.
Other reflective crack control measures that are used to rehabilitate distressed pavements include placing Stress-Absorbing Membrane Interlayers (SAMI), placing grids or fabrics on a surface before placing HMA, break and seat of the pavement, rubblization of the pavement, and reconstruction. Some potential disadvantages with these processes are that they can be cost prohibitive, ineffective, difficult to recycle or difficult to construct. Another disadvantage with these processes is that if the road is not reconstructed, it may still have cracking problems.
In order to solve some of the problems discussed above, interlayers have been created that have the ability to relax stress while maintaining stability. An example of such an interlayer is described in U.S. Pat. No. 6,830,408, which is incorporated by reference in its entirety. Such an interlayer has an aggregate structure that includes a large amount of fine aggregate. More specifically, 6 to 14% of aggregate used in such an interlayer is able to pass through a No. 200 sieve and 100% of aggregate is able to pass through a 9.5 mm sieve. While such an interlayer is impermeable to water so as to prevent surface water from penetrating and collecting below it, it has the disadvantage of being substantially impermeable to vapor. When such an interlayer is placed on Portland Cement Concrete (PCC) or another paved surface, the interlayer has the potential to trap vapor underneath it. As changes occur in climatic and environmental conditions, this causes the PCC to release moisture or vent. The interlayer then rises creating a blister. This causes overlays on top of this interlayer also to rise and blister. Through experimentation, the inventors of the present invention have determined that water vapor is a predominant component of the vapor that creates the blisters.
In order to overcome these disadvantages, a bituminous mixture that is able to form a layer that remains substantially moisture impervious and retains its ability to retard the formation of reflective cracks while having increased vapor permeability is needed. This bituminous mixture should be able to be used to create various layers of a roadway including base layers, interlayers, and overlays.